Rotors in wind turbines are formed with parts that are exposed in operation to strong centrifugal forces, rotational forces as well as other forces such as gravity and those arising from the action of the wind on the blades. The hub and the blades connected thereto are exposed to strong forces and therefore they become severely stressed, especially when in use. Strong forces involved in wind turbine rotors are a serious problem which in most cases give rise to fatigue failure of the attaching means used for blade and hub connection. The parts in a wind turbine rotor, and particularly the blade hub connection, must be therefore of heavy construction. This results in undesirably heavy, large, and expensive parts.
As it is known, wind turbine blades are releasably attached, through their respective blade root portions, to the rotor hub through the use of attaching means. The attaching means include a plurality of bushings and corresponding studs arranged in a portion of the hub and in a portion of the blade root. Studs are threadably received into the bushings for releasable connection of the blades to the hub.
Fatigue failure of studs in wind turbine applications is known, specifically of their securing ends. Such failure is known to be caused by bending stresses (radial forces) and tensile stresses (longitudinal or axial forces) acting on the blade and hub connection.
All forces acting on the blades, that is, both axial forces along the axis of the blades and bending forces to the axis of the blades, result in a pulling force applied on each stud. Since the blades are usually attached to the rotor hub through a bearing by means of screws and the screws of the blades usually do not coaxially correspond to those of the hub, said pulling force gives rise to a bending force on the assembly. Such bending force depends on the above mentioned pulling force and it is a varying force such that additional bending fatigue damages are caused to the studs.
Attempts to provide an effective solution for accommodating high damaging forces on the blades have been proposed in the art. One example is providing the ends of the bushings with spherical bearings as disclosed in U.S. Pat. No. 4,773,824. The rotor of the wind turbine shown in this document includes a rotatable hub carrying a number of blades and attaching means for blade connection to the hub. The attaching means include connecting rods having threaded opposite ends screwed into corresponding ends of the bushings. Between the connecting rod and the bushing a space is defined. The ends of the bushings have respective cup bearings having spherical faces in order to absorb centrifugal forces of the blades.
Although this solution deals with the problem of absorption of centrifugal forces when in use, this is carried out through the provision of bearings which operates with bushing and stud as a whole. Therefore no provision us made for an efficient accommodation of high bending stresses acting on the wind turbine rotor.
US2007065288 discloses a releasable attachment of a wind turbine blade portion to a wind turbine hub by means of bushings embedded into the blade portion. The internal thread of the bushings may extend over the entire length or at least one portion of the bushing has no thread. Therefore, the stud introduced into the bushing and engaged in the thread may be put under tension.
Although according to this solution the blade portion can be connected to the hub independent of the cyclic loads acting on the blade, it however does not face the problem of how to efficiently absorbing the centrifugal forces of the blades on the hub, particularly critical bending stresses resulting from said forces acting on the blade and hub connection.
In general, in the prior art solutions the bushing thread is substantially as long as the stud thread or in some cases shorter. Bushing and stud thread lengths are substantially the same such that a rigid bushing-stud connection is created. Therefore the studs are retained against any bending and axial directions. As used herein, a bending direction stands for any direction where studs can be bent or flexed, such as a radial direction, a tangential direction, and the like, relative to the axis of the stud. On the other hand, an axial direction stands for a direction substantially along the axis of the stud.
Such attaching means provide for a good stiffness for the tensile stresses. However, theses attaching means do not provide for an efficient bending stress accommodation. This is a serious drawback in situations in which bending stresses are higher than tensile stresses and especially when such bending stresses are applied at different times due to the high speed gusts of wind combined with rotation of the blades themselves resulting in fatigue damage.